1. Field of the Invention
The present invention relates to a laser unit used for image formation in a copying machine or the like. More particularly, the present invention relates to a laser unit capable of preventing vibration of a polygon motor and also capable of preventing dust from entering the inside of a polygon cover.
2. Description of the Related Art
FIG. 5 is a sectional view showing an example of a conventional laser unit used for image formation in a copying machine or the like. The laser unit 300 has a polygon motor 20 assembled on a lower housing 10. The laser unit 300 further has a polygon mirror 30, a seal member 80, and a polygon cover 50. The body of the polygon motor 20 is provided with a motor mounting flange 21 for securing the polygon motor 20 to the lower housing 10. The polygon mirror 30 is attached to the rotating shaft of the polygon motor 20. The motor mounting flange 21 is secured to the lower housing 10 with screws 22 in such a manner that the seal member 80 (FIG. 6; thickness=H+ΔH) disposed on the lower housing 10 is sandwiched between the motor mounting flange 21 and the lower housing 10. In this case, each screw 22 is secured to a mounting shaft (not shown) projecting with a predetermined height H from the upper surface of the lower housing 10. The central portion of the seal member 80 is compressed by ΔH in the direction of thickness thereof. The central portion of the seal member 80 prevents the generation and propagation of vibration of the polygon motor 20. The polygon cover 50 is secured to mounting shafts 15 projecting with a predetermined height H from the upper surface of the lower housing 10. At this time, the lower peripheral portion of the polygon cover 50 sandwiches the peripheral portion of the seal member 80 between itself and the lower housing 10 to seal the inside of the polygon cover 50 from the outside, thereby preventing dust from entering the inside of the polygon cover 50.
In the above-described conventional laser unit, the seal member is formed in a single monolithic structure, as shown in FIG. 6, extending continuously from the polygon motor mounting portion to the polygon cover mounting portion. Therefore, if strain is produced in the polygon motor mounting portion by mounting of the polygon motor, the strain has an influence also on the peripheral portion of the seal member. If the polygon cover is mounted in this state, the hermeticity in the polygon cover may be degraded, and strain produced by mounting of the polygon cover may have an influence also on the central portion of the seal member. One approach to solve the above-described problem is to separate the central and peripheral portions of the seal member as shown in FIGS. 7A and 7B. With this technique, the amount of strain decreases. However, the handling and assembly of the seal member are troublesome because it comprises two separate parts. In another conventional example, vibration prevention for the polygon motor is made, but dust prevention for the polygon mirror is effected simply by bringing the polygon cover into close contact with the frame, without using a seal member, as in the invention disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-245442.